1. Field of the Invention
This invention relates to systems which actively absorb noise. More particularly it relates to active systems to absorb noise in HVAC ducts.
2. Prior Art
In some applications in the field of acoustics devices are needed which reflect or absorb acoustical waves in a specified way. One of these applications is in HVAC systems to absorb or block air-born noise produced by the HVAC fans and compressors. At high frequencies this specified behaviour, e.g. absorpion, can be achieved by simple, passive constructive means, i.e. the of use absorptive materials like foam rubber or glass wool, and by giving the non-reflecting surface a special shape. However at low frequencies the dimensions of absorptive structures get large and impractical.
Solutions are known in which an active noise cancellation device is incorporated into the HVAC duct (e.g. Nelson & Elliott, “Active Control of Sound”, ch. 7.8).
It is an objective of this invention to improve the performance of such active noise blockers in HVAC ducts.
An important component which is used in the present invention is Max Hobelsberger's device for simulation of an acoustical impedance (U.S. Pat. No. 5,812,686) which is used to establish a specified acoustical impedance. This device is an active device which uses an electroacoustic transducer which acts, together with a control system, as acoustical impedance.
Another component used in this invention is an element described in U.S. Pat. No. 6,088,459 (Inv. Max Hobelsberger).